The present invention generally relates to a drivetrain for a gas turbine engine and, more specifically, to a drivetrain for using a direct driven generator as a starter motor for a gas turbine engine.
Some aircraft engines are started by devices, external to the aircraft, which supply torque to accelerate the engine up to its self-sustaining speed within a required time. It has long been recognized that to minimize weight, cost and complexity, it is desirable to utilize equipment already on the aircraft, or other gas turbine powered vehicle, for this purpose. A generating system can be used for engine starting by operating a generator as an electric motor.
The above described arrangement is generally referred to as a generator-starter drive. The generator-starter is usually located on a drivetrain connected only to the gas generator spool of a multi-spool engine so that the motor need not accelerate the other spools' inertia during engine startup.
U.S. Pat. No. 4,315,442 to Cordner describes a starter-generator drive using a differential and a hydrostatic transmission. The hydrostatic transmission is controlled to provide an energy-dissipating starting mode when the generator is operating as an electric motor to start the engine aircraft.
U.S. Pat. No. 4,708,030 also to Cordner describes a starter-generator drive having a multi-speed transmission for stepping the input speed from an engine to a hydro-viscous dissipative clutch to provide plural speed ranges of operation and thus limit the amount of slip that occurs within the clutch in each range to provide the constant speed drive of the generator.
Some hybrid vehicle applications require a gas turbine engine's output power be used to directly drive very large generators. As such these applications have the generators rotationally connected to the power turbine spool of the multi-spool engine. As can be seen, there is a need for an improved drivetrain that allows a multi-spool gas turbine engine, having counter-rotating main engine shafts, to directly drive an electrical generator at power turbine speed and, alternatively, use that generator as a starter motor for the engine thereby eliminating the need for a second motor for starting on a drivetrain isolated to the gas generator shaft.